The present invention relates generally to the characterization of semiconductors and more particularly to the characterization of semiconductor materials and devices using an ac surface photovoltage (SPV) method to determine the surface space charge capacitance. The invention is particularly useful in determining parameters such as the surface state density of a semiconductor and/or an oxide/insulator (i.e., an oxide or any other type of insulator,) charge in a dielectric film which may be formed on a semiconductor, either naturally or intentionally (i.e. by thermal oxidation), but, as will hereinafter be pointing out, may be used in determining other parameters of a semiconductor.
As is known the surface state density of a semiconductor is useful, for example, in indicating the quality and contamination of a semiconductor surface or of the interface between the semiconductor and an oxide coating which may be formed on a semiconductor while the oxide/insulator charge is useful in indicating the quality and contamination of the oxide/insulator coating itself.
The surface photovoltage effect as applied to semiconductors and techniques for measuring the (ac) surface photovoltage so as to determine characteristics such as the surface space charge capacitance in general are well known in the art.
Known patents of interest relating to the surface photovoltage effect include U.S. Pat. No. 4,544,887, issued on Oct. 1, 1985 in the name of E. Kamieniecki, which discloses a method of measuring photo-induced voltage at the surface of semiconductor materials (i.e. the surface photovoltage); U.S. Pat. No. 4,286,215, issued on Aug. 25, 1981 in the name of G. L. Miller, which discloses a method and apparatus for the contactless monitoring of the carrier lifetime in semiconductor materials; U.S. Pat. No. 4,333,051, issued on June 1, 1982 in the name of A. M. Goodman, which discloses a method and apparatus for determining minority carrier diffusion length in semiconductors; U.S. Pat. No. 4,433,288, issued on Feb. 21, 1984 in the name of A. R. Moore, which discloses a method and apparatus for determining minority carrier diffusion length in semiconductors; and U.S. Pat. No. 4,663,526, issued on May 5, 1987 in the name of E. Kamieniecki, which discloses a method and apparatus for the nondestructive readout of a latent electrostatic image formed on an insulating material.
Known publications of interest relating to the characterization of semiconductors and/or the surface photovoltage effect in general include Emil Kamieniecki, "Surface Photovoltage Measured Capacitance: Application To Semiconductor/Electrolyte System", J. Appl. Phys. Vol. 54, No. 11, November, 1983, pp. 6481-6487; Emil Kamiencki, "Determination of surface space charge capacitance using a light probe", J. Vac. Sci. Technol., Vol. 20, No. 3, March, 1982, pp. 811-814; Hiromichi Shimizu, Kanji Kinameri, Noriaki Honma and Chusuke Munakata, "Determination of Surface Charge and Interface Trap Densities in Naturally Oxidized n-Type Si Wafers Using ac Surface Photovoltages", Japanese Journal of Applied Physuics, Vol. 26, No. 2, February, 1987, pp. 226-230; A. Ser. Y. H. Tsuo, John A. Moriarty, W. E. Miller and R. K. Crouch, "Si and GaAs Photocapacitive MIS Infrared Detectors", J. Appl. Phys., Vol. 51, No. 4, April 1980, pp. 2137-2148; Olof Engstrom and Annelle Carlsson, "Scanned Light Pulse Technique For the Investigation of Insulator-semiconductor Interfaces", J. Appl. Phys. Vol. 54, No. 9, September, 1983, pp. 5245-5251; E. Thorngren and O. Engstrom, "An Apparatus for the Determination of Ion Drift in MIS Structures", J. Phys. E: Sci, Instrum., Vol. 17, 1984, printed in Great Britain, pp. 1114-1116; E. Kamieniecki and G. Parsons, "Characterization of Semiconductor-Electrolyte System by Surface Photovoltage Measured Capacitance", 164th meeting of the Electrochemical Society, Washington, D. C. October, 1983; R. R. Chang, D. L. Lile and R. Gann, "Remote Gate Capacitance-Voltage Studies for Noninvasive Surface Characterization", Appl. Phys. Lett. Vol. 51, No. 13, Sept. 28, 1987, pp. 987-989; Chusuke Munakata, Shigeru Nishimatsu, Noriaki Honma and Kunihiro Yagi, "Ac Surface Photovoltages in Strongly-Inverted Oxidized p-Type Silicon Wafers", Japanese Journal of Applied Physics, Vol. 23, No. 11, November 1984, pp. 1451-1461; R. S. Nakhmanson, "Frequency Dependence of the Photo-EMF of Strongly Inverted Ge and Si MIS Structures--I. Theory", Solid State Electronics, 1975, Vol 18, pp. 617-626, Pergamon Press, Printed in Great Britain; R. L. Streever, J. J. Winter and F. Rothwarf, "Photovoltage Characterization of MOS Capacitors", Pro. Int. Symp. Silicon Materials Sci & Tech., Philadelphia, May 1977 (Electrochem. Soc. Princeton, 1977) pp. 393-400; R. S. Nakhmanson, Z. Sh. Ovsyuk and L. K. Popov, "Frequency Dependence of Photo-EMF of Strongly Inverted Ge and Si MIS Structures--II Experiments", Solid State Electronics, 1975, Vol. 18, pp. 627-634 Pergamon Press, Printed in Great Britain; Chusuke Munakata and Shigeru Nishimatsu, "Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer", Japanese Journal of applied Physics, Vol 25, No. 6, June, 1966, pp. 807-812; Chusuke Munakata, Mitsuo Nanba and Sunao Matsubara, "Non-Destructive Method of Observing Inhomogeneities in p-n Junctions with a Chopped Photon Beam", Japanese Journal of Applied Physics, Vol. 20, No. 2, February, 1981, pp. L137-L140; Chusuke Munakata and Shigeru Nishimatsu, "Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer", Japanese Journal of Applied Physics, Vol 25, No. 6, June, 1986, pp. 807-812; S. M. Sze, "MIS Diode and Charge-Coupled Device", Physics of Semiconductor Devices, John Wiley & sons Inc. New York 1981, second edition, pp. 362-394.
The front-end of a typical semiconductor device fabrication line involves numerous steps after the initial scrubbing and cleaning of the raw wafer. These steps include oxidation, deposition, masking, diffusion, and implant operations. It can take several weeks from start to finish and testing of the final product. As can be appreciated, process variations which cause yield losses that are detected only at the end of the water fabrication cycle are an economic disaster for manufacturers.
This invention is concerned with a method and apparatus for monitoring contamination and defects of a semiconductor surface (interface) and/or of a dielectric film which may be coating a semiconductor and/or of a device, such as a metal-oxide-semiconductor or a metal-insulator-semiconductor, which includes a layer of semiconductor material. The invention is also applicable to determining the doping type and the doping concentration of a semiconductor in the region adjacent to the (front) surface. One of the most important applications of the technique described in this invention is in connection with silicon device fabrication and in particular monitoring of the oxidation processes used in the fabrication of such devices. However, the technique may also find application in monitoring of other processes such as implantation and diffusion as well as in monitoring processing of semiconductor materials other than silicon, such as for example, gallium arsenide or mercury cadmium telluride.
As will hereinafter be explained, the present invention addresses the use of the (ac) surface photovoltage effect developed under certain specific conditions for the characterization of the bulk and surface (interface) properties of semiconductors. The semiconductor specimen being examined may be bare or may be coated with single layer of dielectric material such as a native oxide (e.g., Si/SiO.sub.2) or a multi-layer dielectric coating (e.g., Si/SiO.sub.2 /polyimide, Si/SiO.sub.2 /Si.sub.3 N.sub.4, etc.) or may be an MIS (metal-insulator-semiconductor) or MOS (metal-oxide-semiconductor) device. More specifically, the present invention makes use of the known fact that the (ac) surface photovoltage signal (the voltage photo-induced at the surface of a semiconductor) when measured under certain defined conditions is proportional to the reciprocal of the semiconductor space-charge capacitance.
The defined conditions of measurement are as follows: (1) the wavelength of the illuminating light is shorter than that corresponding to the energy gap of the semiconductor material, (2) the light is intensity modulated with the intensity of the light and the frequency of modulation being selected such that the induced (ac) voltage signal is directly proportional to the intensity of light and reciprocally proportional to the frequency of modulation.
When the surface of the specimen is illuminated uniformly this relationship maybe expressed as ##EQU1## where .delta.V.sub.s is the surface photovoltage, C.sub.sc is the space charge capacitance. .phi. is the incident photon flux, R is the reflection coefficient of the semiconductor material, f is the modulation frequency of the light, and q is the electron charge. K is equal to 4 for squarewave modulation of light intensity and is equal to 2.pi. for sinusoidal modulation. Details on the derivation of this relationship are presented in the above noted paper by Emil Kamieniecki entitled "Determination of Surface Space Charge Capacitance Using A Light Probe" published in the Journal of Vacuum Science Technology, Vol. 20, No. 3, March 1982, pages 811-814. If the illumination of the semiconductor surface is local and not uniform, .delta.V.sub.s is determined by using the equation .delta.V.sub.m =(s/S).delta.V.sub.s where V.sub.m is the output voltage, s is the area of the illuminated portion (plus diffusion) and S is the total area of the semiconductor. C.sub.sc is then determined using the equation noted above.
U.S. Pat. No. 4,544,887, cited above, describes two specific arrangements for measuring the photo-induced voltage at the surface of a specimen of semiconductor material under the conditions noted above, namely, (i) for a specimen of semiconductor material placed in a suitable electrolyte, and (ii) for a specimen of semiconductor material spaced from the reference electrode by an insulating medium such as a gas or a vacuum. However, each arrangement has its shortcomings. The gas or vacuum arrangement is particularly unsatisfactory because of the electrostatic force of attraction between charges induced on opposing faces of the reference electrode and the semiconductor which tend to deflect the semiconductor towards the reference electrode resulting in nonlinearities in the system and the generation of spurious signals while the electrolyte arrangement will cause changes (contamination) in the surface being tested. U.S. Pat. No. 4,544,887 further suggest that the surface photovoltage so determined may be used to characterize properties of a semiconductor material using "conventional" capacitance analysis. However, no method, conventional or nonconventional, which can be used for actually characterizing semiconductor materials once the surface photovoltage has been detected using the disclosed conditions is actually described in the patent. Similar equations establishing the proportionality between the surface photovoltage and the space charge capacitance along with the relation and conditions of measurement in connection with MIS devices are found in the Sher, etc. article noted above and the Nakhamson article (1975), also noted above. Equation 16 in the Nakhamson article deals with the imaginary component of the surface photovoltage signal.
The present invention, as will hereinafter be shown, describes an arrangement for measuring the surface photovoltage in a way which is useful for characterization of semiconductors, especially, but not limited to, semiconductors in the form of wafers and, in addition, describes in detail a method of actually determining a number of parameters of the semiconductor once the surface photovoltage is so obtained, the method for determining the parameters being different from conventional and known capacitance analysis techniques.
As will hereinafter be pointed out, one of the main features of the present technique for characterizing semiconductors by using low intensity modulated light generated during photovoltage involves the use of the dependence of the photovoltage signal so detected on the bias voltage. Another and very important feature is the way in which the parameters of the semiconductor specimen are derived from this dependence.
Measurements of surface photovoltage (generated due to low intensity illumination) versus bias voltage, in general, are very well known. R. L. Streever, J. J. Winter and R. Rothwarf in an article entitled, "Photovoltage Characterization of MOS Capacitors" published in Proc. Int. Symp. Silicon Materials Sci. & Tech., Philadelphia, May 1977 (Electrochem. Soc., Princeton, 1977) pp. 393-400; A. Sher. Y. H. Tsuo, and John A. Mariarty in an article entitled, "Si and GaAs Photocapacitive MIS Infrared Detectors" published in the Journal of applied Physics Vol. 51, No. 4, April 1980, pages 2137-2148; Olof Engstrom and Annelie Carlsson in an article entitled, "Scanned Light Pulse Technique for the Investigation of Insulator-semiconductor Interfaces" published in the Journal of Applied Physics Vol. 54, No. 9, September 1983, pages 5245-5251; and E. Thorngren and O. Engstrom in an article, "An Apparatus for the Determination of Ion Drift in MIS Structures" published in J. Phys. E: Sci. Instrum., Vol. 17, 1984, pp. 1114-1116 all disclose such measurements.
One of the shortcomings with the system disclosed in the above articles is that they are all limited to MIS or MOS structures. The present invention, on the other hand, is not limited to such structures but rather is applicable (1) to arrangements in which a semiconductor wafer (eventually having a dielectric coating) and an insulator which is used to separate the semiconductor from a conductive electrode for SPV testing are separate elements and (2) to MIS or MOS structures, in which the insulator and the semiconductor are a unitary structure (permanently integrated). From the point of view of the system characteristic and method of characterization, the main difference between the arrangement where the semiconductor and the insulator are a unitary structure and the arrangement where the insulator and the semiconductor are separate elements is that the insulator in the unitary structure is much thinner than the insulator in the non-unitary structure. More specifically, while the insulator thickness in MIS/MOS structures is typically around 1000 A or less, the typical thickness of the separately formed insulator arrangement is typically around 10 .mu.m (about 100 times thicker). Therefore to achieve similar changes in the semiconductor space-charge region using a separately formed (thicker) insulator requires about 100 times higher bias voltage (e.g. around 500 volts as opposed to about 5 volts). Because of this much higher bias voltage, the conventional analysis technique used for capacitance-voltage measurements and used for ac surface photovoltage in MIS/MOS structures cannot be used when a thick insulator is being used.
The conventional approach for capacitance-voltage measurements makes use of the distribution of the bias voltage (V.sub.g) between the insulator (V.sub.i) and the semiconductor (V.sub.s) i.e. (V.sub.g =V.sub.i +V.sub.s), to evaluate the relation between the surface potential V.sub.s and the applied voltage V.sub.g ; for conventional capacitance analysis see Chapter 7 of the book by S. M. Sze, noted bive, for surface photovoltage see page 5248 in the paper by Engstrom et al. noted above. With a 10 .mu.m thick insulating spacer (such as a sheet of Mylar) the bias voltage V.sub.g is hundreds of times higher than surface potential V.sub.s. Consequently, an error in evaluation of the voltage drop across the insulator (V.sub.i) due to e.g., uncertainty in the thickness of the insulating spacer and hence its capacitance C.sub.i (V.sub.i =Q.sub.ind /C.sub.i where Q.sub.ind is the charge induced in the semiconductor) makes evaluation of the surface potential from the applied voltage impractical.
The measurement of the surface photovoltage versus the combination of the incident light and the modulating frequency of the light is shown in U.S. Pat. No. 4,544,887 noted above.
It is also known to determine the capacitance in a semiconductor for characterization purposes by measuring AC current rather than surface photovoltage.
Accordingly, it is an object of this invention to provide a new and improved method for characterizing semiconductor materials (either coated with an insulator or uncoated) and semiconductor devices using the surface photovoltage effect.
It is a further object of this invention to provide a method and apparatus as described above which involves determining the surface space charge capacitance.
It is another object of this invention to provide a method and apparatus as described above which is specifically suited for use with thick insulators but which can also be used, if desired, with thin insulators.
It is still another object of this invention to provide a method and apparatus as described above which may be used for determining surface state (interface trap) density.
It is yet still another object of this invention to provide a method and apparatus as described above which may be used for determining the oxide/insulator charge.
It is a further object of this invention to provide a method and apparatus as described above which may be used for determining doping type.
It is another object of this invention to provide a method and apparatus as described above which may be used for determining doping concentration.
It is still another object of this invention to provide a method and apparatus as described above which is non-invasive.
It is a further object of this invention to provide a method and apparatus for use in characterizing a semiconductor wafer.